The ink-jet recording head of the type described above has been attracting special attention because discharging orifices for discharging recording ink as droplets for example can be arrayed at an extremely high density so that the recording with a high degree of resolution can be obtained; because it is rather simple to fabricate the whole recording head which is compact in size; because the IC technology which has made remarkable advance and are highly reliable in the fabrication of the semiconductor devices and microscopic components can be fully utilized in the fabrication of the recording heads so that a long array of discharging orifices and a flat array (two-dimensioned) of discharging orifices can be easily fabricated, whereby many discharging orifices can be assembled into a recording head at a high degree of density; and because the fabrication of the recording heads, therefore, can be made with a high degree of yield and at less costs.
FIG. 1 illustrates the construction of one of the recording heads of the type described above. A heater board 27 which comprises electro-thermal converting elements or heaters for ink discharging (to be referred to as "discharging heaters" hereinafter) 29 and wires 28 made of aluminum for supplying the electric power to the electro-thermal converting elements 29 is fabricated on the surface of a silicon substrate by a thin film forming process. A top plate 30 which has a plurality of partition walls for defining a plurality of liquid paths 25 is bonded to the heater board 27, whereby an ink-jet recording head can be fabricated.
A recording liquid (ink) is supplied through a supply inlet formed on the top plate 30 into a common liquid chamber 23 and then supplied to each liquid path 25. When the heater 29 is energized, the ink filled in each of the liquid paths 29 generates bubbles so that an ink droplet is discharged from a discharging orifice 26.
However, because of the solidification of the ink, the intrusion of air bubbles into the liquid paths due to the vibration of the recording head or the operation of the recording head at high temperatures and other adverse causes, the recording head used in the ink-jet recording system frequently becomes unable to discharge the liquid droplets. Especially in the case of the system utilizing discharging heaters, the thermal energy is used to discharge the liquid droplets so that the temperature of the recording head tends to rise. In the normal ink discharging condition, almost all the thermal energy is absorbed by the ink droplets to be discharged so that the temperature of the recording head rises to 50.degree.-60.degree. C. at the most, but when the ink discharging operation is interrupted due to the causes described above and the like, the thermal energy generated by the discharging heaters is stored within the head so that its temperature rises higher than 150.degree. C. with the result of the breakdown of the recording head. Especially it must be noted here that since the top plate 30 is in general fabricated by molding a suitable resin so that the top plate 30 starts its deformation at a temperature of the order of 120.degree. C.
Furthermore, even when the interruption of the liquid or ink droplet discharge does not occur because of the reasons mentioned above, thermal energy is accumulated at one or more portions of the discharging heaters due to a long recording period, recording conditions and the like so that the temperature of the recording head sometimes rises in excess of its normal operation temperature.
When the temperature variations become greater in the recording head, the variations in quality of the recorded image formed by the landing of the ink droplets discharged from the recording head whose temperature vary, occur.
In the conventional recording heads, the heater board 27 is equipped with a plurality of heating elements or discharging heaters so that in order to dissipate heat, for instance an aluminum plate is securely bonded to the heater board 27. The heat dissipating plate are equipped with one or more temperature sensors such as thermistors or the like in the vicinity of the heater board 27 so that in response to the outputs of the temperature sensors the temperature of the recording head is controllable. But the difference in temperature exists between the heater board 27 and especially the portions in the vicinity of the discharging heaters 29 and the positions at which the temperature sensors are disposed, and furthermore the transmission of heat through the aluminum plate of course takes time with the result of the time delay of heat transmission so that the correct and quick countermeasure cannot be carried out. As a result, in the case of recording mode, the variations in concentration or tone of the recorded images occur. In addition, the breakdown of the recording head due to an abnormal temperature rise during the interruption of the ink discharge.
It must be noted here especially that in the ink-jet recording systems, the temperature of ink is one of the very important factors to be taken into consideration in view of attaining a high grade recording. The reason is that the physical properties of a recording ink such as its surface tension, a degree of viscosity and the like vary in response to its temperature and as a result the quantity of discharged ink and the supply rate of the ink vary. In view of the above, it is extremely important that the temperature of the ink be maintained within a predetermined range. Therefore, means for maintaining the temperature of the recording liquid within a predetermined range, is required in addition to means used for preventing the abnormal temperature rise, so that some recording systems are equipped with suitable heating means, that is, one or more heaters for warming the recording head at the predetermined range (to be referred to as "warming heaters" hereinafter).
So far such the heater or heaters are disposed on a suitable member or members as like the temperature sensors, but the warming heater or heaters the recording head through the member or members so that the variations in time of heat transmission result and therefore a satisfactory degree of energy efficiency cannot be attained.
Furthermore, in the conventional recording systems, the temperature sensors and the heaters must be disposed independently so that their costs and the costs for assembling them become expensive and consequently the recording heads themselves also become expensive.
A further problem is that it is next to impossible to control the temperature gradient within the heater board once it happens.
It follows therefore that, as disclosed in Laid-open European Patent Application Publication No. 353925/1990, the temperature sensors and warming heaters for maintaining the temperature of the heater board within a predetermined temperature range be disposed integrally over the heater board. According to this construction, the temperature sensors and the heaters are disposed in the vicinity of the discharging heaters for so that the temperature control with a high degree of accuracy and response is expected to be carried out.
However, according to this teaching, the positional relationships between the discharging heaters, the warming heaters for warming the recording head and the temperature sensors have not been so far taken into consideration so that no countermeasure against the temperature distribution which vary rapidly within the heater board can be taken. As a result, it is difficult to control the ink discharging characteristics at a high degree of accuracy in practice. More specifically, the temperatures of the discharging heaters which mostly influence the recording liquid discharging characteristics are dependent upon the heat from the warming heaters which are statically controlled and the temperature changes caused by the discharging heaters themselves whose heat generation dynamically changes. That is, the temperatures of the discharging heaters are dependent upon the complicated combinations of the thermal energy generated by both the discharging and warming heaters. As a result, the stabilized recording liquid discharging characteristics cannot be assured only by the control for maintaining the temperature within a predetermined range in response to the outputs from the temperature sensors whose positions are not taken into consideration in design and construction.